Keiko Tomoe is Hotaru's
mother. When Hotaru was just 8 years old, a fire broke out in the
Mugen area, where they lived, and killed Keiko. When she died, she was 32 years old. Few
pictures exist of her, including a tiny one in Volume 09, and one in a later flashback.
While she was living, she wore a crystal called the Tioron Crystal. After her death,
Professor Tomoe,
her husband, gave it to Hotaru to sooth her pains. That crystal eventually became the power
source of the Deathbusters, who wished to make it stronger so that they could take over
the world.